When a pneumatic tire having a radial structure is subjected to a load, bending moments act on the bead portions in accompaniment with deformation in which the bead portions collapse outward in the tire axial direction in the regions on the tire radial direction outer sides of the rim flanges. Because of this, tensile force acts on the tire inner sides of the bead portions and compressive force acts on the tire outer sides of the bead portions taking the bending neutral axis as a boundary. The compressive force acts on the cords of the turn-back portions of the carcass ply turned back around the bead cores and produces compressive strain, and when the compressive strain becomes excessive, there are cases where it leads to fatigue fracture of the cords of the carcass ply.
Conventionally, pneumatic tires for construction vehicles have been proposed in which the durability of the bead portions is improved by suppressing compressive strain of the cords of the turn-back portions and suppressing fatigue fracture of the turn-back portions (e.g., see patent citations 1 and 2).
In the pneumatic tires of patent citations 1 and 2, compressive strain produced in the cords of the turn-back portions is suppressed and fatigue fracture of the cords of the turn-back portions is suppressed by gradually decreasing the inter-cord distance between the body portion and the turn-back portions of the carcass outward in the tire radial direction from the bead cores until the inter-cord distance first reaches a minimum value and then gradually increasing the inter-cord distance until the inter-cord distance reaches a maximum value, to thereby optimize the distance and so forth from the baseline of the applicable rim to the positions at which the inter-cord distance reaches the minimum value and the maximum value.
In the compressed regions on the outer sides of the bending neutral axis, shear strain and compressive strain decrease in proportion to the distance from the outer surfaces of the bead portions, so in the conventional art, local concentration of compressive strain in the turn-back portions is avoided and cord fracture is suppressed by placing, in proximity to the bending neutral axis, a zone in which the compressive strain of the turn-back portions becomes concentrated and which becomes the center of occurrence of cord fracture.